U.S. patent number 6,078,208 [Application Number 09/085,828] was granted by the patent office on 2000-06-20 for precision temperature sensor integrated circuit.
This patent grant is currently assigned to Microchip Technology Incorporated. Invention is credited to Hartono Darmawaskita, James B. Nolan.
United States Patent |
6,078,208 |
Nolan , et al. |
June 20, 2000 |
Precision temperature sensor integrated circuit
Abstract
A precision temperature sensor produces a clock frequency which
varies predictably over wide variations of ambient temperature. The
invention has a oscillation generator, two independent current
generators, a reference oscillator and a frequency counter. The
outputs of the two independent current generators are combined to
provide an approximately linear capacitor charging current which is
directly proportional to changes in temperature. The capacitor
charging current is used to drive the oscillation generator which
outputs a clock frequency that is approximately linearly dependent
on temperature with determinable slope and intercept. The frequency
counter compares the output of the oscillation generator with the
independent reference oscillator to compute a digital value for
temperature. The precision temperature sensor is implemented on a
single, monolithic integrated circuit.
Inventors: |
Nolan; James B. (Chandler,
AZ), Darmawaskita; Hartono (Chandler, AZ) |
Assignee: |
Microchip Technology
Incorporated (Chandler, AZ)
|
Family
ID: |
22194215 |
Appl.
No.: |
09/085,828 |
Filed: |
May 28, 1998 |
Current U.S.
Class: |
327/512;
374/E7.035 |
Current CPC
Class: |
G01K
7/01 (20130101) |
Current International
Class: |
G01K
7/01 (20060101); H01L 035/00 () |
Field of
Search: |
;327/512,513
;331/66,176 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tran; Toan
Attorney, Agent or Firm: Katz; Paul N. Chichester; Ronald L.
Frohwitter
Claims
What is claimed is:
1. A precision temperature sensor circuit comprising:
an oscillation generator;
a first current generator coupled to the oscillation generator;
a second current generator coupled to the oscillation
generator;
wherein an output current from the first current generator is
combined with an output current from the second current generator
for achieving a combined current which is linearly proportional to
temperature; and
wherein the circuit is implemented on a single, monolithic
integrated circuit.
2. The circuit in accordance with claim 1 further comprising:
a frequency counter coupled to the oscillation generator; and
a reference oscillator coupled to the frequency counter.
3. The circuit in accordance with claim 1 wherein the oscillation
generator comprises:
a flip-flop;
a comparator circuit coupled to the flip-flop;
a reference voltage circuit coupled to the comparator circuit;
at least one capacitor coupled to the comparator circuit;
at least one switch circuit coupled to the at least one capacitor;
and
at least one inverter coupled to the flip-flop.
4. The circuit in accordance with claim 3 wherein the reference
voltage circuit is a bandgap reference voltage circuit.
5. The circuit in accordance with claim 3 wherein the comparator
circuit is comprised of two comparators.
6. The circuit in accordance with claim 5 wherein one input of each
of the two comparators and the first current generator are all
coupled to the reference voltage circuit.
7. The circuit in accordance with claim 5 wherein a first of the
two comparators is for setting the flip-flop.
8. The circuit in accordance with claim 5 wherein a second of the
two comparators is for resetting the flip-flop.
9. The circuit in accordance with claim 3 wherein the at least one
switch coupled to the at least one capacitor comprises:
a first plurality of switches coupled to a first capacitor; and
a second plurality of switches coupled to a second capacitor.
10. The circuit in accordance with claim 9 wherein the first and
second plurality of switches are MOSFET transistors.
11. The circuit in accordance with claim 9 wherein the flip-flop
comprises:
a first output for providing a first clock frequency directly
proportional to temperature variation and for controlling the first
plurality of switches; and
a second output for providing a second clock frequency directly
proportional to temperature variation which is the complement of
the first stable clock frequency and for controlling the second
plurality of switches.
12. The circuit in accordance with claim 9 wherein the first
plurality of switches provides a charging path and a discharging
path for the first capacitor.
13. The circuit in accordance with claim 9 wherein the second
plurality of switches provides a charging path and a discharging
path for the second capacitor.
14. The circuit in accordance with claim 12 wherein the discharging
path for the first capacitor is connected to an input of a first
comparator by one of the first plurality of switches.
15. The circuit in accordance with claim 13 wherein the discharging
path of the second capacitor is connected to an input of a second
comparator by one of the second plurality of switches.
16. The circuit in accordance with claim 1 wherein the combined
current is used by the oscillation generator to produce a clock
frequency which varies proportionally to temperature.
17. The circuit in accordance with claim 1 wherein the output
current from the second current generator is combined with the
output current from the first current generator for adjusting the
slope and intercept of the combined current with respect to
temperature.
18. The circuit in accordance with claim 1 wherein the first
current generator comprises:
a first bias generator;
a first current mirror coupled to the first bias generator for
producing a first output current that varies proportionally with
temperature.
19. The circuit in accordance with claim 18 wherein the first bias
generator comprises:
a first amplifier circuit;
at least one transistor coupled to the first amplifier circuit;
and
at least one resistor having a small positive temperature
coefficient coupled to an input of the first amplifier circuit.
20. The circuit in accordance with claim 18 wherein the current
mirror is comprised of a plurality of transistors which are
individually selectable to produce the first output current.
21. The circuit in accordance with claim 18 wherein one of the at
least one resistor is external to the single, monolithic integrated
circuit.
22. The circuit in accordance with claim 1 wherein the second
current generator comprises:
a second bias generator;
a second current mirror coupled to the second bias generator for
producing a second output current that varies proportionally with
temperature.
23. The circuit in accordance with claim 22 wherein the second bias
generator comprises:
a second amplifier circuit;
a first bias circuit for producing a first bias voltage wherein the
first bias voltage is coupled to a first input of the second
amplifier circuit; and
a second bias circuit for producing a second bias voltage wherein
the second bias voltage is coupled to a second input of the second
amplifier circuit.
24. The circuit in accordance with claim 23 wherein the first bias
circuit for producing the first bias voltage is comprised of at
least one resistor having a small positive temperature
coefficient.
25. The circuit in accordance with claim 24 wherein one of the at
least one resistor is external to the single, monolithic integrated
circuit.
26. The circuit in accordance with claim 23 wherein an output of
the second amplifier circuit is coupled to the oscillation
generator for producing a reference voltage.
27. The circuit in accordance with claim 22 wherein the second
current mirror is comprised of a plurality of transistors which are
individually selectable to produce the second output current.
28. A precision temperature circuit comprising:
an oscillation generator;
a first current generator coupled to the oscillation generator for
producing a first output current that varies proportionally with
temperature;
a second current generator coupled to the oscillation generator for
producing a second output current that varies proportionally with
temperature and is of opposite slope with respect to temperature
from the first output current; and
wherein the circuit is implemented on a single, integrated
circuit.
29. The circuit in accordance with claim 28 wherein the first
output current is combined with the second output current for
achieving a combined current which varies proportionally to
temperature.
30. The circuit in accordance with claim 29 wherein the output
current from the second current generator is combined with the
output current from the first current generator for adjusting slope
and intercept of the combined current with respect to
temperature.
31. The circuit in accordance with claim 29 wherein the output
current from the second current generator is combined with the
output current from the first current generator for adjusting slope
and intercept of the clock frequency with respect to
temperature.
32. The circuit in accordance with claim 28 wherein the first and
second current generators may be programmed to compensate for
process and supply voltage variations.
33. A precision temperature sensor circuit comprising:
a plurality of oscillation generators;
at least one bias generator and at least one current mirror;
and
wherein the at least one current mirror is coupled to the at least
one bias generator for producing an output current that varies
proportionally with temperature.
34. The circuit in accordance with claim 33 further comprising a
reference voltage circuit coupled to the plurality of oscillation
generations.
35. The circuit in accordance with claim 33 further comprising a
frequency counter coupled to the plurality of oscillation
generators.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to integrated circuits which
measure ambient temperature. Specifically, the present invention is
a precision temperature sensor that produces an approximately
linear capacitor charging current, which in turn is used to produce
a clock frequency that is predictable with variances in ambient
temperature. The invention incorporates a relaxation oscillator,
two independent current generators, a reference oscillator and a
frequency counter to compute ambient temperature. The invention is
implemented on a single, monolithic integrated circuit.
2. Description of the Prior Art
The prior art is described by FIG. 1 which shows a typical
integrated temperature sensor based on the well known
.DELTA.V.sub.BE model that generates a voltage proportional to
temperature. The prior art integrated circuit temperature sensors
require a differential amplifier and an analog to digital (A-to-D)
converter to convert a voltage from a diode, thermistor or other
source into a digital temperature equivalent. The traditional,
voltage oriented A-to-D converter may also require unnecessary
hardware and software overhead not needed for temperature sensing
applications. Furthermore, the prior art circuit requires a
precision reference voltage which can be very costly depending on
the requirements for accuracy.
Therefore, a need existed to provide a temperature sensor
implemented on an integrated circuit which is capable of accurate
measurement of temperature variations and which reduces complexity,
board space and/or die area and pin count.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a temperature
sensor that does not rely on a voltage oriented A-to-D converter,
but instead incorporates a current to frequency A-to-D circuit.
It is another object of the present invention to provide a
temperature sensor which does not require a precise voltage
reference.
It is another object of the present invention to provide a
temperature sensor with reduced cost to manufacture, die area and
pin count.
In accordance with one embodiment of the present invention, a
temperature sensor that produces an accurate measurement over wide
variations of ambient temperature is disclosed. The precision
relaxation oscillator is comprised of an oscillation generator, a
first current generator for producing a first output current, a
second current generator for producing a second output current, a
reference oscillator and a frequency counter. The invention is
implemented on a single, monolithic integrated circuit.
In accordance with another embodiment of the present invention the
reference oscillator is comprised of a second oscillation
generator.
In accordance with another embodiment of the present invention the
reference oscillator is comprised of a crystal oscillator.
In accordance with another embodiment of the present invention one
or more external resistors may be coupled to either the first or
second current generators to produce the respective output
currents.
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following, more particular,
description of the preferred embodiments of the invention, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the prior art showing a
temperature sensor circuit.
FIG. 2 is a block diagram of the present invention.
FIG. 2A is a graph depicting the linear characteristics and the
relationships between the current parameters of the present
invention.
FIG. 3 is a block diagram of the Proportional to Absolute
Temperature (PTAT) current generator found in the present
invention.
FIG. 4 is a block diagram of the Complementary to Absolute
Temperature (CTAT) current generator found in the present
invention.
FIG. 5 is a block diagram of the moderate precision temperature
sensor embodiment of the present invention.
FIG. 6 is a timing diagram of specific parameters of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2, a precision temperature sensor 1 that produces
an accurate measurement over wide variations of ambient temperature
is shown.
The precision temperature sensor 1 is comprised of an oscillation
generator 100, a first current generator 300 which is typically a
Proportional to Absolute Temperature (PTAT) current generator, a
second current generator 200 which is typically a Complementary to
Absolute Temperature (CTAT) current generator, a reference
oscillator 400 and a frequency counter 500. However, those skilled
in the art will recognize that the roles for the current generator
may be reversed, i.e. the first current generator 200 may be the
CTAT and the second current generator 300 may be the PTAT. In the
preferred embodiment of the present invention, the precision
temperature sensor 1 is implemented on a single, monolithic
integrated circuit.
Referring to FIGS. 2 and 2A, the independent CTAT 200 and PTAT 300
current generators provide a CTAT current 290 and a PTAT current
390 which are approximately linear with respect to temperature. In
the preferred embodiment, the CTAT current 290 and PTAT current 390
(FIGS. 4 and 5) are combined to form a capacitor charging current
(I.sub.CCC) 190 (I.sub.CCC 190=CTAT current 290+PTAT current 390)
which has a constant, positive slope with respect to
temperature.
In the preferred embodiment the PTAT current 390 is the dominant
component of I.sub.CCC 190 and has a constant, positive slope
directly proportional to temperature, but which may vary slightly
from device to device because of fabrication anomalies or process
drift. The CTAT current 290 has a slope, which is proportional to
temperature and is combined with the PTAT current 390 so that a
predictable, uniform I.sub.CCC 190, which varies approximately
linearly with temperature, is achieved from device to device. In
the preferred embodiment CTAT current has a negative slope with
respect to temperature. In other embodiments, the PTAT current 390
may be the sole component of I.sub.CCC 190 and may be digitally
trimmed as described below. In still other embodiments, the CTAT
current 290 may be the sole component of I.sub.CCC 190 and may also
be digitally trimmed. I.sub.CCC 190 is subsequently used by the
oscillation generator 100 (FIG. 2) to produce a predictable clock
frequency that varies with temperature.
In the preferred embodiment as illustrated in FIG. 2, the
oscillation generator 100 is comprised of a set-reset flip-flop
160, a comparator circuit 180 further comprised of two comparators
182 and 184, two capacitors 110 and 120, four transistor switches
130, 132, 134 and 136, two inverters 140 and 142 and a bandgap
reference voltage circuit 150 for producing a reference voltage
152. Those skilled in the art will recognize that other
combinations of these components or similar components are possible
to achieve substantially the same results.
The transistor switches 130 and 134 provide charging paths for the
capacitors 110 and 120, respectively. The transistor switches 132
and 136 provide discharging paths for the capacitors 110 and 120,
respectively. In the preferred embodiment, the transistor switches
130, 132, 134 and 136 are MOSFET transistors, however, those
skilled in the art will recognize that the invention is not limited
to this technology.
The oscillation generator 100 operates by having one capacitor
charge while the other capacitor discharges. The discharge path for
the capacitor 110 is connected via transistor switch 132 to an
input of the comparator 182. The discharge path for the capacitor
120 is connected via transistor switch 136 to an input of the
comparator 184.
In the preferred embodiment, and for best performance, a stable
reference voltage source such as a bandgap reference voltage
circuit 150 is used. The bandgap reference voltage circuit 150
provides a single reference voltage 152, which is connected to
second inputs of comparators 182 and 184, and is used to set the
common mode voltage at each comparator 182 and 184. The P.sub.BIAS
input 325 for the bandgap reference voltage circuit 150 is an
output of the PTAT bias generator 310 which is described below. The
bandgap reference voltage circuit 150 has the advantage of
stabilizing I.sub.CCC 190 and minimizing the error due to variance
in comparator input slew and propagation delay. Furthermore, in
order to cancel the effects of reference voltage drift, the CTAT
200 current generator relies on the same reference voltage 152 as
the comparators 182 and 184. There are various embodiments of the
bandgap reference voltage circuit 150 which are well known to those
skilled in the art. However, the novel way in which the bandgap
reference voltage circuit 150 is implemented in the present
invention is not disclosed by the prior art.
The output of comparator 182 is connected to the set input 162 of
the flip-flop 160. The output of comparator 184 is connected to the
reset
input 164 of the flip-flop 160. Thus, as the capacitors 110 and 120
alternatively charge and discharge, the outputs of the comparators
182 and 184 alternatively set and reset the flip-flop 160 thus
producing a clock output.
The Q output 166 of the flip-flop 160 provides a clock frequency
that is directly related to temperature variation. In the preferred
embodiment, the Q output 166 is also coupled to transistor switch
132 and via inverter 140 to transistor switch 130. Thus, the Q
output 166 provides the signal that controls the transistor
switches 130 and 132, which in turn open and close the charging and
discharging paths for capacitor 110.
The complementary Q output 168 of flip-flop 160 provides a second
clock frequency that is also directly related to temperature and
the complement of Q output 166. The complementary Q output 168 is
coupled to transistor switch 136 and via inverter 142 to transistor
switch 134. Thus, the complementary Q output 168 provides the
signal that controls the transistor switches 134 and 136 which in
turn open and close the charging and discharging paths for
capacitor 120.
The reference oscillator 400, has a temperature coefficient much
smaller than that of the current generators 200 and 300, thereby
providing a stable clock frequency over wide variations in
temperature. The reference oscillator 400 and the Q output 166 of
flip-flop 160 are coupled to the frequency counter 500. For
moderate precision, the reference oscillator 400 may be a second
oscillation generator 100 as described below.
For higher precision, a crystal oscillator is the preferred
embodiment because it has high stability over temperature. The
frequency counter 500 compares the temperature sensitive Q output
166 with the temperature insensitive reference oscillator 400 and
provides an output 510 that is an accurate representation of
temperature. Frequency counters of various embodiments are well
know to those skilled in the art and therefore need not be
described further.
Referring to FIG. 3, wherein like numerals reflect like elements,
the PTAT current generator 300, known to those skilled in the art
as a .DELTA.V.sub.BE circuit, is comprised of PTAT bias generator
310 and a PTAT current mirror 350 for producing a PTAT current
390.
The PTAT bias generator 310 is comprised of an amplifier circuit
320, a first bias circuit 330 for producing a first bias voltage
across a selectable resistor with a small linear temperature
coefficient 332 or 334 and a second bias circuit 340 for producing
a second bias voltage. The present invention provides that an
external resistor 334 may be selected over the internal resistor
332 by a select bit 336.
The first and second bias voltages provide the inputs to the
amplifier 320. The output of the amplifier 320 is P.sub.BIAS 325
which is coupled to the first 330 and second 340 bias circuits, the
PTAT current mirror 350 and the bandgap reference voltage generator
150 (FIG. 1).
The PTAT current mirror 350 is comprised of a plurality of
transistors 352 from one to n. As shown in FIG. 3, the control
electrodes of the transistors 352 are coupled to P.sub.BIAS 325.
Trimming for the proper output current is performed digitally by
programmable selecting one or more of the current mirror
transistors 352 via the calibration select 354 to obtain the
desired PTAT current 390. In the preferred embodiment, the
calibration select switches 354 are PMOS transistors. However,
those skilled in the art will recognize that other types of
switches are possible which result in substantially the same
result.
In the preferred embodiment, the current mirror 350 acts as a
current divider which is well known to those skilled in the art. In
other embodiments the current mirror 350 may be configured as a
current multiplier. The PTAT current 390 is the sum of the selected
outputs from the current mirror transistors 352. Thus, the PTAT
current generator 300 permits determination of the slope and
intercept of the approximately linear PTAT current 390 with respect
to temperature as shown in FIG. 2A.
Referring to FIG. 4, wherein like numerals reflect like elements,
the CTAT current generator 200 is comprised of a CTAT bias
generator 210 and a current mirror 250 for producing the CTAT
current 290.
The CTAT bias generator 210 is comprised of an amplifier circuit
220, at least one resistor with a small positive temperature
coefficient 232 and 234 for regulating the input current to the
amplifier and a transistor 240 for providing an input current to
the amplifier 220. The amplifier 220 is a cascode configuration for
supply and noise rejection. The reference voltage 152 is coupled to
an input of the amplifier 220. The present invention provides that
an external resistor 234 may be selected over the internal resistor
232 by a select bit 236.
The current mirror 250 is comprised of a plurality of transistors
252 from one to n. As shown in FIG. 4, the control electrodes of
the transistors 252 are coupled to the output of the CTAT bias
generator amplifier 220. Trimming the CTAT current 290 for
achieving the proper PTAT:CTAT balance is performed digitally by
programmable selecting one or more of the current mirror
transistors 252 via the calibration select 254 to obtain the
desired CTAT current 290. In the preferred embodiment, the
calibration select switches 254 are PMOS transistors. However,
those skilled in the art will recognize that other types of
switches are possible which result in substantially the same
result.
Thus, trimming is the process of controlling the output currents
290 and 390 of the current mirrors 250 and 350, respectively, with
respect to temperature. When the trimmed output currents 290 and
390 are combined at the oscillation generator 100 (FIG. 2), the
proper slope and intercept (current versus temperature) for
I.sub.CCC 190 is achieved for the purpose of producing a clock
frequency 166 (FIG. 2) that predictably varies with temperature.
The approximately linear I.sub.CCC 190 produces an approximately
linear clock frequency 166 with respect to temperature. Therefore,
it follows that the trimmed output currents 290 and 390 are
combined as I.sub.CCC 190 to control the proper slope and intercept
of clock frequency 166 over temperature as illustrated in FIG.
2A.
In the preferred embodiment, the current mirror 250 acts as a
current divider which is well known to those skilled in the art. In
other embodiments the current mirror 250 may be configured as a
current multiplier. The CTAT current 290 is the sum of the selected
outputs from the current mirror transistors 252.
Referring to FIG. 5, wherein like numerals reflect like elements
and numbers with primes (') represent additional circuitry of like
characteristics, the moderate precision temperature sensor
embodiment is shown. In this embodiment a common PTAT bias
generator 310 and CTAT bias generator 210 each drive a unique pair
of PTAT current mirrors 350 and 350' and CTAT current mirrors 250
and 250', respectively. The PTAT bias generator 310 also drives the
reference voltage generator 150, as previously described. The
PTAT/CTAT current mirrors 350 and 250 produce temperature sensitive
output currents 390 and 290. These PTAT/CTAT currents 390 and 290
drive the oscillation generator 100 to produce a clock frequency
that varies proportionally with temperatures as described
above.
The PTAT/CTAT current mirrors 350' and 250' are very similar to the
PTAT/CTAT current mirrors 350 and 250, except that PTAT/CTAT
current mirrors 350' and 250' produce temperature insensitive
output currents 390' and 290' by controlling the ratio of PTAT/CTAT
current 390' and 290' so that the slope of the combined current is
insensitive with respect to temperature. These PTAT/CTAT currents
390' and 290' drive the oscillation generator 100', which is
structurally similar to oscillation generator 100. The output of
oscillation generator 100' is a clock frequency which does not
vary, or varies very little, with respect to temperature. The
frequency counter 500 compares the two clock frequencies from the
oscillation generators 100 and 100' to produce a digital
temperature measurement, as described above.
Referring to FIG. 6, wherein like numerals reflect like elements, a
timing diagram for the temperature sensor 1 is shown. V1 112
reflects the charging and discharging of capacitor 110 (FIG. 1).
Note that the positive slope (charging) of V1 112 is equal to
I.sub.CCC 190 divided by the capacitance of capacitor 110. The
maximum amplitude of V1 112 is equal to the reference voltage 152.
CMP1 reflects the output of the comparator 182 which is coupled to
the set input 162 of the flip-flop 160.
V2 122 reflects the charging and discharging of capacitor 120. In
this case the positive slope of V2 122 is equal to I.sub.CCC 190
divided by the capacitance of capacitor 120. CMP2 reflects the
output of the comparator 184 which is coupled to the reset input
164 of the flip-flop 160. CLK is the Q output 166 of the flip-flop
160.
For a 50 percent duty cycle, the values of capacitors 110 and 120
are identical which result in similar slopes for V1 112 and V2 122.
As the capacitor voltage exceeds the reference voltage 152, the
respective comparator 182 and 184 pulses low which causes the
flip-flop 160 to change state. RST (reset) is used to initialize
the comparators 182 and 184 and the flip-flop 160 to a known state
and to operate the sensor in a one shot mode.
The present invention produces a clock frequency which is directly
proportional to temperature. This is accomplished by providing bias
currents which when trimmed and summed yield a predictable
capacitor charging current that is approximately linearly
proportional to temperature. The circuit 1 performs digital
trimming of the bias currents 290 and 390 via the programmable
current mirrors 250 and 350 to eliminate process variations, using
a stable voltage reference such as a bandgap reference voltage
circuit 150. The circuit 1 produces a temperature sensitive clock
166 via a dual capacitor, dual comparator oscillation generator
100. The frequency counter 500 compares the output of the
oscillation generator 100 to the output of a reference oscillator
to compute a digital temperature 510. Also, analog design
techniques, well known to those skilled in the art, such as
component matching and cascode current sources enhance the
stability of the circuit.
Although the invention has been particularly shown and described
with reference to a preferred embodiment thereof, it will be
understood by those skilled in the art that changes in form and
detail may be made therein without departing from the spirit and
scope of the invention.
* * * * *